Zinc, vitamin C, vitamin D, old-fashioned chicken soup. Chances are you’ve probably tried all of these remedies to ward off symptoms of a cold, but scientists now say that you can cross at least one of them off your list.
In the most rigorous study to date investigating whether vitamin D can protect against colds, researchers found that healthy adults who took large doses of the supplement were no less likely to come down with upper respiratory infections — including colds, flu and sinus infections — than those taking placebo.
The new study, led by Dr. David Murdoch and his colleagues at the University of Otago in New Zealand, followed 322 healthy participants who were randomly assigned to take either vitamin D or a placebo on a monthly basis for 18 months. All participants came to the research facility so that scientists could observe them taking the pills, but neither the volunteers nor the scientists knew which pills contained the vitamin and which were placebo. Those getting vitamin D supplements received 200,000 IU for each of the first two months, and then 100,000 IU monthly for the remainder of the trial. That amounts to more than 3,000 IU a day, when health officials currently recommend that most adults get about 600 IU of vitamin D daily (primarily to maintain bone health and avoid the bone-weakening disease of osteoporosis).
By the end of the study, participants in the vitamin D group had developed a cold or flu an average 3.7 times, compared with 3.8 times in the placebo group. Taking vitamin D also had no impact on the severity or duration of people’s symptoms nor did it reduce the number of workdays they missed.
Scientists started looking at vitamin D for cold prevention after early studies showed that tuberculosis patients who had genes that efficiently metabolized vitamin D were less prone to serious lung infections. There was also the intriguing observation that people who lived in sunnier climates — and therefore presumably had higher levels of vitamin D, since the skin makes the vitamin when exposed to sunlight — also tended to have fewer colds than those who lived in areas with less exposure to the sun.
Previous clinical trials have led to similarly disappointing results, however. But the authors of the current study note that people who are deficient in vitamin D, which is thought to contribute to immune function, may yet benefit from taking supplements. Studies involving children in Mongolia, for example, where the harsh climate prevents much sun exposure, showed that vitamin D supplements helped lower respiratory infections by 50%.
As for the protective effect of extra vitamin D in healthy adults who already have normal levels: “Whether vitamin D can prevent colds is pretty much done and dusted in healthy people,” says Murdoch. “Now we need to focus on other groups.”
Recent studies also show the supplement may not be as helpful as hoped in preventing other diseases. For example, rigorous trials suggest that taking vitamin D in low-dose supplements (400 IU) doesn’t help healthy postmenopausal women avoid bone fractures — but higher doses may still be helpful, and the supplements do help prevent bone breaks in women living in nursing homes or assisted-living facilities. Also, taking vitamin D does not lower the risk of developing breast cancer.
That’s not to say that maintaining vitamin D levels isn’t necessary for good health overall: the vitamin is needed for the body to soak up bone-building calcium from foods. Good dietary sources of D include dairy products such as yogurt, eggs, cheese and fortified milk. Getting at least a few minutes of sunlight a day is also enough for the body to make adequate vitamin D, though experts note that increased public health warnings about the risk of skin cancer from sun exposure may be leading to more cases of D deficiency than doctors think.
Whether or not such people may benefit from taking supplements during cold and flu season, Murdoch’s latest findings suggest that for the majority of healthy individuals, chicken soup may have to do for now.
The new study was published in the Journal of the American Medical Association.